1. Field of the Invention
The present invention relates to a choke coil, which is used in an interleaved PFC (Power Factor Correction) circuit, which has two coil windings, and which can act as two virtually independent choke coils.
2. Description of the Related Art
In recent years, a PFC circuit has been set in a power supply of an electronic device. Especially in an application over 300W, an interleaved PFC circuit has been adopted in order to reduce ripple current and power loss. “Interleave” means connecting circuits in parallel, with their phases shifted from each other.
FIG. 13 shows an example of an interleaved PFC circuit. The interleaved PFC circuit is configured so that two circuits, one of which includes a choke coil L1, a switching element S1 and a diode D1, and the other of which includes a choke coil L2, a switching element S2 and a diode D2, are connected in parallel and are shifted in phase from each other. A capacitor COUT for ripple riddance is connected in parallel with a load resistance RLOAD in an output side of the interleaved PFC circuit. Input voltage VIN is, for example, a full-wave rectified AC 100V from a commercial power supply.
FIGS. 14A to 14D are waveform charts at each point of the interleaved PFC circuit shown in FIG. 13. FIG. 14A shows a timing of on-off of the switching elements S1, S2. FIG. 14B shows electric currents IL1, IL2 flowing through the choke coils L1, L2 and an input electric current IIN (sum of the electric currents IL1, IL2). FIG. 14C shows electric currents I1, I2 flowing through the diodes D1, D2. FIG. 14D shows electric current ICOUT (=(I1+I2)−IOUT (IOUT: output electric current)) flowing through the capacitor COUT.
As shown in FIG. 14D, in the interleaved PFC circuit, frequency of a ripple current is twice as high as switching frequency, so that the ripple current reduces effectually.
Generally, above mentioned interleaved PFC circuit needs two independent choke coils, but it costs high and requires large space for mounting. Therefore, a choke coil of 2 in 1 structure for interleave has been desired.
The first patent document, Japanese Patent Application Laid-Open No. 2006-60108 proposes a transformer of 2 in 1 structure. However, the transformer is not for a PFC circuit but a high-voltage transformer for lighting a backlight of a liquid crystal display device. In the transformer, two pairs of first and second windings are on one assembly of magnetic cores of E-I-E-shape so that the transformer can act as virtually two high-voltage transformers. In this structure, magnetic fluxes generated by the two pairs of first and second windings pass through an I-shaped core between end surfaces of a pair of E-shaped cores, and the magnetic fluxes are of the same direction in the I-shaped core so that the magnetic fluxes are added to each other in the I-shaped. Therefore, there is a problem that a sectional area of the I-shaped core needs to be large, namely, the I-shaped core needs to be thick, and a shape of the assembly of magnetic cores needs to be large.
As above mentioned, using two independent choke coils in an interleaved PFC circuit costs high and requires large space for mounting, and even by the 2 in 1 structure disclosed in the first patent document, a shape of the assembly of magnetic cores is not always sufficiently small.